In the past few years, some researchers have explored whether warfare and societal collapse might be explained in part by swings in climate. But what about the opposite effect? Can humanity's skirmishes change the climate?

A 2007 study found that periods of cold weather preceded 12 of 15 major conflicts in China's ancient dynasties. The frost would have created food shortages, the study suggested, which would have inspired rebellions and made communities more vulnerable to invasion. More recently, a study in Science argued that dramatic shifts in climate would have affected agriculture, contributing to the fall of the Roman Empire.

But what about the opposite effect? Can humanity's skirmishes change the climate?

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Yes, says a new study in The Holocene by Julia Pongratz of the Carnegie Institution for Science. It all comes down to a trade-off between people and trees: when a brutal war or devastating plague significantly reduces a human population, forests have the chance to re-grow and absorb carbon dioxide, mitigating the greenhouse effect.

Pongratz reconstructed global land cover from 800 AD to the present and modelled the carbon cycle for the same time period in order to test how land usage influenced climate change. She found, for example, that during the Mongol invasions in Asia (1200 – 1380), which some historians estimate killed at least 15 million people, newly flourishing trees in once deforested areas inhaled nearly 700 million tons of carbon from the atmosphere – an amount equivalent to the world's total annual demand for gasoline today.

There was also significant re-growth during the conquest of the Americas (1519 – 1700), in which native populations were decimated by up to 90 per cent – but not during shorter events like the Black Death (1347 – 1400) and the fall of the Ming Dynasty in China (1600 –1650).

However, Pongratz points out, any sighs of relief forests drew from human war and death failed to overcome the climate damage caused by a long and continuing history of deforestation. As far as the Earth is concerned, there have been way too many people, and too few trees, for far too long.

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